Ethnic minority rebels in South Kordofan have been fighting government forces for three years in a largely-hidden war which the United Nations says has affected more than one million people.

Sudan's National Intelligence and Security Service (NISS), which has authority over Rapid Support, invited journalists to Daldako following accusations that a sister Rapid Support unit in Sudan's western region of Darfur had abused civilians.

In an April report, UN chief Ban Ki-moon said Rapid Support elements had attacked and burned villages in Darfur.

After Sudanese opposition leader Sadiq al-Mahdi reportedly made similar comments, NISS agents arrested him on May 17.

Observers have said Mahdi's arrest undermined a dialogue between the ruling National Congress and other political parties aimed at ending Sudan's multiple wars, economic crisis and political divisions.

Mahdi's arrest was a power play by elements of the security service, said Magdi El Gizouli, a fellow at the Rift Valley Institute.

"In presenting itself as a fighting force capable of challenging insurgencies, the NISS is obviously boosting its stature with the ambition to achieve the kind of political recognition that the SAF continues to enjoy," Gizouli said last week.

The Sudanese Armed Forces (SAF) are distinct from the military units of NISS.

An AFP reporter who visited Daldako on Tuesday found Rapid Support troops digging trench defences.

More than 1,000 men were spread across a three-kilometre zone in Daldako with tanks, artillery, anti-aircraft guns and recoilless rifles.